The present invention is in the field of catalysts used in the catalytic reduction of nitrogen oxides (NOx) using lower hydrocarbons.
Nitrogen oxide (NOx) reduction with hydrocarbons has recently attracted attention due to stricter environmental regulations. Pollution of the atmosphere by NOx emission from combustion sources continues to be a major environmental concern. NOx emissions are linked to acid rain, increased ground-level ozone concentrations that lead to urban smog, respiratory problems, adverse effects on terrestrial and aquatic ecosystems, and depletion of the stratospheric ozone layer.
Using methane as a reducing agent has certain advantages since it is the least expensive lower hydrocarbon and exists abundantly in natural gas. There have been numerous studies in the literature on NO reduction over ZSM-5 supported catalysts (zeolite supported) using hydrocarbons including methane. In some studies precious metals such as palladium were incorporated in to the zeolite using the ion exchange method. Pd2+ exchanged H-ZSM-5 and Pdxe2x80x94Hxe2x80x94Ce-ZSM-5 display considerable activity for NO reduction using methane in the presence of oxygen. Acidity is essential for this reaction to take place.
Ceria is widely recognized as a type of oxygen scavenging material. Many studies on three way catalysts can be found in the literature. Investigation of NO+CO reactions over alumina supported rhodium catalysts doped with ceria or ceria supported rhodium catalysts have been studied extensively. Using the interaction between Pd and Ce, new catalyst designs have been developed.
Recently, many researchers have focused on the use of ceria. Zhang and Stephanopoulos in J. Catal. 164 (1996) 131, investigated the effect of the addition of Ce(II) ions on the activity and hydrothermal stability Cu-ZSM-5 for NO decomposition. They found that the presence of cerium suppressed CuO particle formation and dealumination, providing higher hydrothermal stability for zeolite structure and higher copper dispersion.
Along with the reaction/mechanistic studies, thermodynamic properties of cerium and lanthanum compounds containing oxygen and sulfur have been studied by Kay et al. (see J. Alloys and Compounds, 192 (1993) 11). Characterization of Pt/xcex3-alumina containing ceria has been extensively studied by Shyu et al. (see J. Catal., 114 (1988) 23 and J. Catal, 115 (1989) 16) which provides insight to the effect of Pd and Pt on the reduction/oxidation of ceria.
In an effort to improve the oxygen storage/transport characteristics, Cho (J. Catal, 131 (1991) 74) has incorporated gadolinia onto a commercial ceria powder. Dziewiecka et al. (Catal. Today, 17 (1993) 121) studied NO reduction with hydrocarbons over a series of copper-gadolinia oxide catalysts supported on thermal resistant carriers. They both found that NO reduction activity was increased by the presence of gadolinia.
There are several reports on the literature concerning the use of active carbons as supports for different catalysts. Stegenga et al. (Carbon, 30 (1992) 577) looked into the stability of carbon-supported catalysts in such environments. Several methods to increase the resistance to oxidation were considered. One method was to pre-treat the carbon at high temperatures (2000xc2x0 C.), which caused a decrease in active site density and consequently resulted in higher oxidation resistance. On the other hand, treatment with boron or phosphorus compounds did not result in satisfactory protection of the carbon against oxygen. A very successful method was to cover the interior surface of the carbon with a Si compound, which is transformed into silica and subjected to heat treatment in an inert gas. Gasification of the support was prevented due to the formation of SiC, which is highly resistant to oxidation. Besides retaining a considerably high surface area (600-700 m2/g), this support greatly improved its mechanical strength.
In another study, Gao et al. (Catalysis Letters, 59 (1996) 359) investigated the influence of acid treatments of active carbons on NO reduction over carbon supported copper oxides. A remarkable discovery is the fact that the conversions of NO reduction show a strong dependence on surface oxygen containing groups on the active carbons, and that carboxylic and lactonic sites noticeably favor the NO reduction. Consequently, the conversions in the NO reduction over CuO supported on activated carbon depend strongly on the kind of acid pretreatment of the carbons. Concentrated nitric acid can increase the amount of lactonic and carboxylic sites on the carbon surfaces, in contrast to hydrochloric acid, that leads to a remarkable decomposition of those sites.
Salas-Peregrin et al. (Applied Catalysis B, 8 (1996) 79) looked into the NO reduction over carbon-supported palladium catalysts. According to these researchers, the presence of very low amounts of Pd gives active and stable catalysts towards the NOxe2x80x94CO reaction. It is reported that the presence of CO lowers the consumption of the carbon support by NO.
Pore and surface properties of activated carbon supports can be significantly affected by acid treatments. Wang and Lu (Carbon, 36 (1998) 283) found that acid treatment generally caused an increase in surface area and pore volume of carbon supports because the acid removed impurities. In particular, these authors used Ni2+ supported on activated carbons. Although this study focused on physical properties of the supports, catalytic activity tests were also carried out on methane reforming with carbon dioxide, showing that the activity of this catalysts is greatly influenced by the acid treatment of the support.
It is therefore an object of the current invention to develop a supported catalyst for the reduction of nitrogen oxides (NOx) that may be modified using lanthanide metals such as Ce, Gd, La, and Yb in an effort to increase the oxygen tolerance. Additionally, activated carbon (A.C.) may be added to the catalyst support to yield a catalyst with better catalytic properties than those catalysts supported on titania or activated carbon only.
This invention relates to a catalyst prepared and used in the catalytic reduction of nitrogen oxides (NOx) using lower hydrocarbons. The invention relates to a supported catalyst comprised of at least one active metal and at least one promoter metal attached to a support.
The active metal is comprised of palladium or platinum and mixtures thereof and is present in a range of 0.1% to 30% and preferably in the range of 0.1% to 5%.
The promoter metal is selected from lanthanide metals and mixtures thereof and is present in a weight range of 0.1% to 20% and preferably in the range of 0.1% to 5%.
The ratio of active metal to promoter metal is preferably 2:1.
The support comprises titanium dioxide. The catalyst may additionally contain activated carbon (A.C.). When the support is titanium dioxide together with activated carbon the preferable ratio of titanium dioxide to activated carbon is about 3:1.
The catalysts of the present invention may be prepared using a wet impregnation technique or a modified sol-gel method. When using the modified sol-gel method the catalyst may be prepared in a solution. The solution may be any solvent that can dissolve titanium alkoxides. The solution may be selected from water, alcohol, hexane, benzene or other polar-aprotic solvents to name a few. If alcohol is used, it may comprise pure or mixed alcohols selected from the group consisting of methanol, ethanol, propanol, iso-propanol, and butanol. The active metal-containing precursor, as used herein, may be palladium acetate. The promoter metal-containing precursor, as used herein, may be gadolinium nitrate or cerium nitrate.
The titanium alkoxide may include Ti(OR)4, where R can be CH3, C2H5, linear C3H7, branched C3H7 (isopropyl) or C4H9. It is preferably titanium isopropoxide or titanium oxide.
The invention also includes a method of using the catalyst in the reduction of nitrogen oxides (NOx) using lower hydrocarbons. As used herein, lower hydrocarbon is any compound with C1-6 and is preferably selected from the group consisting of alkanes, alkenes, and alkynes. More preferably selected from the group consisting of methane, ethane, propane, ethylene and propylene. And most preferably methane.
The present invention is to a method of reducing nitrogen oxides (NOx) with at least one lower hydrocarbon using a catalyst, the method comprising the steps of: a) obtaining a supported catalyst, the catalyst comprising at least one active metal and at least one promoter metal attached to a support, wherein the at least one active metal comprises palladium, the support arising from the polymerization of at least one precursor thereof, the at least one active metal and the at least one promoter metal having been attached to the support by the at least one active metal and the at least one promoter metal having been co-precipitated with the precursor of the support; and b) bringing nitrogen oxides and lower hydrocarbon into contact with the catalyst for sufficient time and at sufficient temperature so as to allow nitrogen oxide to be reduced.
The active metal, promoter metal, support and lower hydrocarbon have all been defined above.